The present invention relates to a refractory metal silicide target, a method of manufacturing the target, a refractory metal silicide thin film, and a semiconductor device, and more specifically, to a refractory metal silicide target, a method of simply manufacturing the target, a refractory metal silicide thin film, and a semiconductor device capable of reducing the generation of particles in sputtering and forming a thin film of high quality by densifying or fining a mixed structure and making a uniform composition and further achieving high density and high purification.
A sputtering method is employed as one of the effective methods of forming a refractory metal silicide thin film used for a gate electrode, source electrode, drain electrode of semiconductor devices such as MOS, LSI devices and the like and for wiring. The sputtering method, which is excellent in mass-productivity and the stability of a formed film, is a method such that argon ions are caused to collide with a disc-shaped refractory metal silicide target and discharge a target constituting metal which is deposited as a thin film on a substrate disposed in confrontation with the target. Consequently, the property of the silicide thin film formed by sputtering greatly depends upon the characteristics of the target.
Recently, as a semiconductor device is highly integrated and miniaturized, it is required that a sputtering target used to form a refractory metal silicide thin film produces a less amount of particles (fine grains). That is, since particles produced from a target during sputtering have a very fine grain size of about 0.1-10 xcexcm, when the particles are mixed into a thin film being deposited, they cause a serious problem that the yield of semiconductor devices is greatly reduced by the occurrence of short circuit between wires of a circuit and insufficient opening of wires. Thus, the reduction of an amount of particles is strongly required.
Since it can become effective means to miniaturize a target structure, that is, to make the size of MSi2 grains and free Si grains as small as possible in order to reduce an amount of particles produced from a target, there are conventionally proposed various manufacturing methods of miniaturizing the structure.
For example, Japanese Patent Application Laid-Open No. Sho 63(1988)-219580 discloses that a high density target having a fine structure and containing a small amount of oxygen can be obtained in such a manner that a mixed powder obtained by mixing a high purity refractory metal powder with a high purity silicon powder is subjected to a silicide reaction in high vacuum and a semi-sintered body is formed, then the resultant semi-sintered body is charged into a pressure-tight sealing canister without being crushed and the pressure-tight sealing canister is sintered by a hot isostatic press after having evacuated and sealed. In this case, the thus obtained target has a fine structure having the maximum grain size of MSi2 not greater than 20 xcexcm and the maximum grain size of free Si not greater than 50 xcexcm and containing oxygen not greater than 200 ppm with a density ratio not less than 99%.
Further, Japanese Patent Application Laid-Open No. Hei 2(1990)-47261 discloses that a high density target with a fine structure can be obtained in such a manner that a mixed powder of a high purity refractory metal powder and a high purity silicon powder is subjected to a silicide reaction in high vacuum and a semi-sintered body is formed, then the semi-sintered body is crushed to not greater than 150 xcexcm and further added and mixed with a high purity silicon powder and charged into a pressure-tight sealing canister, then the pressure-tight sealing canister is sintered by a hot isostatic press after having evacuated and sealed. In this case, the thus obtained target has a maximum grain size of MSi2 not greater than 20 xcexcm and a density ratio not less than 99% with only free Si existing in a grain boundary.
Recently, as a semiconductor device is highly integrated and miniaturized, a high purity target containing a very small amount of impurities, which deteriorate the characteristics of the semiconductor device, is required as a sputtering target used to form a refractory metal silicide thin film. In particular, it is strongly required to minimize an amount of oxygen in a target because oxygen, which concentrates on the interface between a silicide layer and an under layer and increases a film resistance, delays signals and lowers the reliability of the device.
Since it is effective oxygen reducing means to make deoxidation by heating a semi-sintered body as a material in vacuum and volatilizing oxygen in the form of silicon oxide (SiO or SiO2), the following manufacturing methods of reducing oxygen are conventionally proposed.
For example, Japanese Patent Application Laid-Open No. Sho 62(1987)-171911 obtains Mo silicide or W silicide each containing a small amount of oxygen in such a manner that a mixed powder obtained by mixing a Mo powder or W powder with a Si powder is heated in vacuum at a temperature less than 800-1300xc2x0 C. and a Mo silicide powder or W silicide powder is synthesized, then the resultant powder is held in vacuum at 1300-1500xc2x0 C. to remove oxygen as SiO by excessive Si.
On the other hand, a trial for optimizing the grain size of a material powder and hot pressing conditions from a view point that the condensation of free Si results to an increase of particles produced and the following manufacturing method is proposed.
For example, Japanese Patent Application Laid-Open No. Sho 63(1988)-74967 obtains a target from which condensed silicon is removed in such a manner that a mixed powder obtained by adding a synthesized silicide powder of xe2x88x92100 mesh with a silicon powder of xe2x88x9242 mesh is heated to 1300-1400xc2x0 C. while applying a preload of 60-170 kg/cm2, then pressed with a pressing pressure of 200-400 kg/cm2 and held after being pressed.
Further, Japanese Patent Application Laid-Open No. Sho 64(1989)-39374 obtains a target from which condensed silicon is removed in such a manner that two types of synthesized silicide powders of xe2x88x92100 mesh having a different composition are prepared and a mixed powder adjusted to have an intended composition is hot pressed under the same conditions as above.
There is a problem, however, that when all the amounts of a mixed powder necessary to form a single target is subjected to a silicide synthesis at once in high vacuum in the above conventional manufacturing methods, resulting MSi2 grains are rapidly grown and coarsened as well as cracks are made to an entire semi-sintered body by a rapidly increased temperature in a silicide reaction because the silicide reaction is an exothermic reaction, and when the semi-sintered body is sintered by pressing in the state as it is, a resultant sintered body cannot be used because the cracks remain.
There is also a problem that since a mixed material powder overflows from a vessel by the rapid increase of temperature in the silicide reaction and a composition is out of an intended composition due to the volatilization of very volatile Si. Thus, when the semi-sintered body is sintered by pressing in the state as it is, a target having a desired composition cannot be obtained.
Further, there is a problem that even if a semi-sintered body is crushed and made to a powder, since hard MSi2 particles which have been grown once and coarsened remain without being finely crushed, a target having a uniform and fine structure cannot be obtained as well as an amount of contamination caused by impurities is increased by crushing and in particular an amount of oxygen is greatly increased.
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. Sho 62(1987)-171911, when a mixed powder is subjected to a silicide synthesization at 800-1300xc2x0 C. and further deoxidized by being heated to high temperature so as to reduce impurity oxygen, there is a problem that since the sintering property of a resultant semi-sintered body is excessively improved, the semi-sintered body cannot be sufficiently crushed in a subsequent crushing process and formed to a segregated structure in which MSi2 and Si are irregularly dispersed, and in particular, when a heating temperature reaches a temperature region exceeding 1400xc2x0 C., this tendency is made more remarkable.
Although a semi-sintered body is crushed in an atmosphere replaced with Ar (argon gas) to prevent an increase of an oxygen content, it is difficult to completely prevent the contamination by oxygen when the semi-sintered body is crushed. Further, a problem also arises in that when a crushed powder is taken out from a vessel such as a ball mill or the like, the powder surely adsorbs oxygen to increase oxygen contained therein, and as a result a finely crushed powder has an increased surface area and an amount of oxygen adsorbed by the powder is greatly increased.
On the other hand, even if a synthesized powder was hot pressed while applying a preload of 60-170 kg/cm2 thereto according to the methods of Japanese Patent Application Laid-Open No. Sho 63(1988)-74967 and Japanese Patent Application Laid-Open No. Sho 64(1989)-39374, condensed silicon was disadvantageously produced and a target having a fine and uniform structure could not be obtained.
Further, when a synthesized powder was hot pressed without being applied with a preload, MSi2 grains obtained by synthesization was grown as well as a composition has an inclined distribution in a target and it was difficult to obtain a target having a fine and uniform structure.
Japanese Patent Application Laid-Open No. Sho 62(1987)-70270 discloses a refractory metal silicide target having a density ratio not less than 97%. Further, Japanese Patent Application Laid-Open No. Sho 62(1987)-230676 discloses a methods of manufacturing a refractory metal silicide target and describes that a target is molded by compacting using a single axis under the conditions of high temperature, high vacuum and high pressing pressure.
However, the above respective prior arts describe only that a target is made by subjecting a material powder for the target to hot pressing and no description is made as to a fine and uniform structure. Thus, these prior arts cannot achieve an object for effectively suppressing particles.
On the other hand, International Patent Application published according to PCT (No. WO91/18125 discloses a silicide target having 400xc3x97104 pieces of silicide with a grain size of 0.5-30 xcexcm existing in a cross section of the mixed structure of the target of 1 mm2 with the maximum grain size of Si not greater than 30 xcexcm and further a silicide target with the average grain size of silicide of 2-15 xcexcm and the average grain size of Si of 2-10 xcexcm.
Since the manufacturing method described in the prior art is insufficient to obtain a fine uniform target structure, the object to suppress the occurrence of particles cannot be sufficiently achieved.
An object of the present invention is to provide a high density and purity refractory metal silicide target which has a fine mixed structure and a uniform composition as well as contains a less amount of impurities such as oxygen and the like, a method of manufacturing the target, a refractory metal silicide thin film and a semiconductor device.
As a result of a zealous study why particles are generated, the inventors of this invention have obtained the following knowledge for the first time:
(1) since free Si has a sputtering rate larger than that of MSi2, as sputtering proceeds, MSi2 is exposed on an erosion surface and MSi2 grains having a weak bonding force with adjacent grains are liable to be removed from the erosion surface, and in particular very fine MSi2 grains remarkably exhibit this tendency;
(2) although the form of erosion in a free Si portion exhibits a wave-shape, as the Si portion increases, the distal end of the wave-shape is made acute and further the height of the wave-shape increases, thus the distal end of Si is dropped off or lacked by the thermal fluctuation in sputtering so that Si is liable to become particles; and
(3) when pores remain in the interface between MSi2 and free Si of a target or in the interior of free Si, projections are formed around the pores, and abnormal electric discharge occurs in the portion where the projections exist in sputtering, by which the projections are dropped or lacked and made to particles, and the like.
Further, the inventors have found it is very effective to suppress the generation of the particles that:
(1) a fine mixed structure is formed such that the number of MSi2 grains (M: refractory metal) which independently exist on any arbitrary surface or in a cross section of 0.01 mm2 of the mixed structure is not greater than 15, MSi2 has an average grain size not greater than 10 xcexcm and free Si existing in the gaps of MSi2 has a maximum grain size not greater than 20 xcexcm;
(2) the mixed structure is arranged such that a Si/M atom ratio X in 1 mm2 of the mixed structure has a dispersion of Xxc2x10.02 and free Si is uniformly dispersed; and
(3) a density ratio is not less than 99.5% over the entire surface of a target, and the like.
Further, the inventors have found that the growth of MSi2 grains produced can be suppressed and a large dislocation (dispersion of a composition ratio) of a composition can be prevented without volatilizing almost all the Si in such a manner that when silicide is synthesized once in a silicide synthesizing process, mixed powders each divided to a small amount of lot are charged into a compacting mold, that is, a depth of the compacting mold to which the mixed powders are charged is set to not deeper than 20 mm and the mixed powders are heated in vacuum and synthesized.
Further, to reduce impurities in a target and increase its purity, the inventors have found that:
(1) a refractory metal silicide semi-sintered body containing a less amount of oxygen not greater than 200 ppm which cannot be obtained by prior art can be obtained in such a manner that a semi-sintered body obtained by synthesizing silicide is crushed once and a resultant crushed powder is deoxidized by being heated in vacuum or in a pressure-reduced hydrogen atmosphere in stead of deoxidizing the semi-sintered body by heating it in the state as it is;
(2) when a plurality of powder charging vessels each having the same inside diameter are prepared and crushed powders are deoxidized so that semi-sintered bodies can be sintered in a shape as they are by a hot isostatic press method or the like, since the semi-sintered bodies have the same shape, a plurality of semi-sintered bodies can be sintered at the same time and there is an advantage that the productivity of targets can be improved;
(3) when the silicide synthesis is performed in a vacuum furnace using a graphite heater and insulator, a semi-sintered body obtained by the synthesization is mixed with carbon and iron and contaminated by them. In contrast, when the silicide synthesis is performed in a vacuum furnace using a heater and an insulator each composed of a high purity refractory material, the contamination can be effectively prevented; and
(4) contamination caused by impurities contained in a material can be effectively prevented by crushing a semi-sintered body in a ball mill having a ball mill main body the inside of which is lined with a high purity material and crushing mediums (balls) formed of a high purity material, and the like.
Further, as a result of a zealous study of hot pressing conditions effected by using a synthesized powder, the inventors have found that the size of MSi2 grains produced is different depending upon a temperature for applying a pressing pressure and how the temperature is increased and that the composition in a target has an inclined distribution in accordance with the temperature and pressure conditions. More specifically, the inventors have found that when a synthesized powder is heated up to just below an eutectic temperature and then applied with a pressing pressure, MSi2 grains formed by synthesization are regrown and that free Si flows in the direction of the end of a target and its composition has an inclined irregular distribution as the MSi2 grains grow.
Further, the inventors have obtained the knowledge that when a certain degree of a pressing pressure is applied at a temperature step less than 1200xc2x0 C. and then heating is effected stepwise or at a low rate up to just below an eutectic temperature and further a larger pressing pressure is applied, the growth of MSi2 grains is effectively prevented, the composition in a target is made uniform and a density of the target is increased for the first time.
The present invention has been completed based on the above knowledges.
More specifically, a refractory metal silicide target according to the present invention is characterized by comprising a fine mixed structure composed of MSi2 (where M: refractory metal) grains and Si grains, wherein the number of MSi2 grains independently existing in a cross section of 0.01 mm2 of the mixed structure is not greater than 15, the MSi2 grains has an average grain size not greater than 10 xcexcm, whereas free Si grains existing in the gaps of the MSi2 grains have a maximum grain size not greater than 20 xcexcm. Specifically, W, Mo, Ti, Ta, Zr, Hf, Nb, V, Co, Cr, Ni are used as the metal (M) constituting the above metal silicide (MSi2).
Note, the shape and the number of MSi2 grains and Si grains in the above mixed structure are measured as follows. That is, the maximum grain size, average grain size and number of MSi2 grains are measured in such a manner that a photograph showing the structure of a target sintered body is obtained by photographing a fracture surface of the sintered body under a scanning type electron microscope (SEM) at a magnification ratio of 1000 and thus obtained photograph is then analyzed with an image analyzer. A visual field to be image-analyzed must cover 10 points.
On the other hand, the maximum grain size, average grain size and number of free Si grains and chain-shaped (link-formed) Si grains are measured in such a manner that a photograph showing the structure of a target sintered body is obtained by photographing a polished surface of the sintered body under a scanning type electron microscope (SEM) at a magnification ratio of 1000, then the photograph is analyzed with an image analyzer. In that case, 5 cross sections obtained by equally dividing the polished surface in the thickness direction thereof at a pitch of 10 xcexcm were measured and when Si grains are freed from other Si grains, they are regarded as free Si, whereas when Si grains are coupled with other Si grains at any portion thereof, they are regarded as chain-shaped Si. A visual field must cover 20 points in each cross section.
Since Si is more deeply eroded than MSi2 by sputtering in the above mixed structure, preferable is a structure arranged such that MSi2 grains are coupled each other like a chain and Si grains exist in the gaps of the MSi2 grains to reduce particles generated in a target because MSi2 grains are liable to be removed or dropped from an eroded surface in a portion where MSi2 independently exists in Si phase.
When the size of MSi2 grains is increased, Si is selectively scattered from MSi2 and forms projections like grains. Since these projections are released and made to particles, the average grain size of MSi2 is preferably not greater then 10 xcexcm and more preferably not greater than 5 xcexcm to prevent the occurrence of the projections. On the other hand, Si is eroded to a wave-shape by sputtering, and as the size of Si is increased, the wave-shape is made acute and deep and Si is liable to be lacked or dropped off. Thus, the maximum grain size of Si is preferably not greater than 20 xcexcm, more preferably not greater than 15 xcexcm, and further more preferably not greater than 10 xcexcm.
When the average value of a Si/M atom ratio in an entire target is assumed to be X, it is preferable that the dispersion of the Si/M atom ratio in an arbitrary cross section of 1 mm2 in the mixed structure is preferably set within the range of Xxc2x10.02. That is, when MSi2 and Si irregularly disperse even if a target has a fine structure, in particular when free Si is locally concentrated and irregularly distributed, since the structure in the target is greatly changed as well as a plasma electric discharge is unstably carried out and particles are induced, the dispersion of the Si/M atom ratio X in an area of 1 mm2 is preferably Xxc2x10.02 and more preferably Xxc2x10.01.
It is preferable to form a high density silicide target in which the density ratio of a target is not less than 99.5% over the entire target. When there remain many pores (holes) due to an insufficient density of a target, the pores exist in an interface between MSi2 and Si or in the interior of Si, projections are formed around the pores in sputtering, an abnormal electric discharge is caused in the portion of the projections and the projections are broken and released by the discharge, which results in the occurrence of particles. Thus, the pores must be reduced as few as possible, and for this purpose, the density ratio of target is preferably not less than 99.5%, more preferably not less than 99.7% and further more preferably not less than 99.8% over the entire target.
It is preferable that a content of oxygen as an impurity is set to not greater than 200 ppm and a content of carbon as an impurity is set to not greater than 50 ppm. When oxygen is taken into a deposited thin film by sputtering a target containing oxygen, silicon oxide is formed in the interface of the thin film and a resistance of the film is increased by the silicon oxide. Thus, to further reduce the resistance of the film, an oxygen content in target is preferably set to not greater than 200 ppm and more preferably not greater than 100 ppm. Further, since carbon also increases a resistance of the film by forming silicon carbide, a carbon content in target is preferably set to not greater than 50 ppm and more preferably not greater than 30 ppm to reduce the resistance of the film.
The contents of iron and aluminium as impurities are set to not greater than 1 ppm, respectively. When iron and aluminium are mixed into a deposited thin film, a deep level is formed in the interface of the thin film and causes a leakage in connection, by which a semiconductor is poorly operated and its characteristics are deteriorated. Thus, an iron content and aluminium content in target are preferably set to not greater than 1 ppm, respectively and more preferably not greater than 0.5 ppm, respectively.
Next, a method of manufacturing a refractory metal silicide target according to the present invention will be described below.
In a process I (step I), a refractory metal powder having a maximum grain size not greater than 15 xcexcm is blended with a silicon powder having a maximum grain size not greater than 30 xcexcm such that a Si/M atom ratio (value X in MSix) is 2-4 and these powders are sufficiently mixed each other in a dry state using a ball mill, V-type mixer or the like so that the silicon powder uniformly disperses in the refractory metal powder. The irregularly mixing of them is not preferable because the structure and composition of a target is made irregular and characteristics of the film formed by using the target are deteriorated. The powders are preferably mixed in a vacuum of not higher than 1xc3x9710xe2x88x923 Torr or in an inert gas atmosphere such as an argon gas to prevent contamination by oxygen. In particular, when a pulverizer or powder crushing mixer such as a ball mill or the like is used, contamination by impurities can be effectively prevented by performing mixing operation in a dry state using a ball mill having a main body the inside of which is lined with a high purity material not less than 5N (99.999%) and crushing mediums (balls) composed of a high purity material so that contamination caused by impurities from a crusher main body can be prevented.
The same material as the refractory metal (M) constituting a target is preferably used as the above high purity material and, for example, W, Mo, Ti, Ta, Zr, Hf, Nb, V, Co, Cr, Ni etc. are used.
As a method of lining the pulverizer main body with the high purity material, there can be employed a method of lining a high purity material sheet, a method of integrally forming a high purity material layer on the inner surface of a main body by various depositing methods such as CVD, plasma vapor deposition, and the like.
The refractory metal powder and silicon powder used as a target material preferably contain impurities, which deteriorate characteristics of a semiconductor device, in an amount as small as possible and preferably have a purity not lower than 5N (99.999%). Further, since coarse powders coarsen formed MSi2 grains and si grains and lowers the dispersing property of Si, the refractory metal powder preferably has a gain size not greater than 15 xcexcm and the silicon powder preferably has a grain size not greater than 30 xcexcm. Further, the refractory metal powder preferably has a grain size not greater than 10 xcexcm and the silicon powder preferably has a grain size not greater than 20 xcexcm. Furthermore, the refractory metal powder preferably has a gain size not greater than 5 xcexcm and the silicon powder preferably has a grain size not greater than 10 xcexcm.
A reason why the value X of the Si/M atom ratio is limited to 2xe2x89xa6Xxe2x89xa64 is as described below. That is, when the value X is less than 2, free Si reduces and further disappear in a silicide target and the structure defined by the present invention cannot be obtained. On the other hand, when the value X exceeds 4, since free Si continuously exists, there is obtained a structure in which MSi2 grains exist in a Si matrix. Consequently, the structure of the present invention that MSi2 grains are coupled each other like a chain and Si grains exist in the gaps of the MSi2 grains is difficult to be obtained. Further, when the value X is less than 2, since a large tensile strength is produced in a formed silicide film, the close contact property of the film with a substrate is deteriorated and the film is liable to be exfoliated or peeled from the substrate. On the other hand, when the value X exceeds 4, since a film resistance increases, a resultant film is improper as an electrode wiring film. Further, when a mixed powder having the value X not less than 2 is synthesized to silicide, since free Si exists, there is an advantage that a crushing property is improved in a process III to be described below.
Si is preferably blended in an amount which is a little in excess of the amount of an intended composition by taking an loss caused by the volatilization of a Si and SiO2 film covering the surface of Si powders into account.
A process II is a process for synthesizing refractory metal silicide as well as forming a semi-sintered body by charging the mixed powder prepared in the process I into a compacting mold and heating the powder in high vacuum or in an inert gas atmosphere. In the process II, since an amount of the mixed powder to be charged into the compacting mold and subjected to a synthesizing operation effected once affects the size of MSi2 grains to be produced and an amount of Si to be volatilized, it is preferable to set an amount of the mixed powder charged once to a depth not higher than 20 mm. When the depth of charge exceeds 20 mm, formed MSi2 grains are coarsened due to a temperature increase caused by a silicide reaction and the powder may be caused to overflow from the vessel by an explosive reaction. On the other hand, when the mixed powder to be charged into the vessel has a depth not higher than 1 mm, the number of vessels used for a single target is greatly increased as well as an amount of production per a synthesizing treatment is greatly reduced, and productivity is lowered. Thus, a preferable depth of charge is 1-10 mm. When Mo is used as a refractory metal powder, however, an amount of the mixed powder to be charged into a vessel is preferably set to a depth not higher than 10 mm and more preferably a depth not higher than 5 mm because a particularly high calorific value is generated by a silicide reaction.
A vessel used here is preferably composed of a high purity Mo, W, Ta, Nb material or the like to prevent the contamination of the mixed powder caused by impurities generated from the vessel and thermal deformation. Further, it is preferable to use the same metal material as a refractory metal (M) constituting intended refractory metal silicide. Further, the flat portion of the vessel may be set to such a shape and size as to enable the vessel to be inserted into calcining equipment such as sintering furnace.
As a heating pattern, it is preferable to effect heating stepwise from a temperature 200xc2x0 C. lower than a silicide reaction start temperature to suppress the growth of MSi2 grains and minimize the change of a composition. A temperature increasing width is preferably 20-200xc2x0 C. That is, when the temperature increasing width is less than 20xc2x0 C., a long time is needed to synthesization and productivity is lowered, whereas the width exceeds 200xc2x0 C. MSi2 grains are grown and the powder is caused to overflow from the vessel by an abrupt increase of temperature, and a composition is changed and the interior of the furnace is contaminated. Further, each temperature is preferably held for 0.1-3 hours. When the holding time is less than 0.1 hour, the temperature of the powder in the vessel is not made uniform and a temperature difference abruptly increases, and MSi2 grains are coarsened. On the other hand, when the holding time exceeds 3 hours, a long time is needed for synthesization and productivity is lowered. Note, the temperature increasing width is preferably set to 20-200xc2x0 C. and more preferably to 50-100xc2x0 C. and the holding time is more preferably set to the range of 0.5-2 hours. In particular, when the temperature is increased to a temperature of 100xc2x0 C. or more higher than the silicide reaction start temperature, it is preferable to set a long holding time at the silicide reaction start temperature or within the start temperature +50xc2x0 C. and the holding time is preferably not shorter than 1 hour. The silicide reaction start temperature can be determined by detecting when a degree of vacuum in the furnace is lowered by the volatilization of Si or silicon oxide (SiO or SiO2) caused by a reaction heat.
Further, the same effect can be achieved by carrying out heating operation slowly in place of the stepwise heating. In this case, a heating rate is preferably controlled to 5xc2x0 C./minute or less. When the heating rate is excessively large, MSi2 grains are grown as well as the powder is caused to overflow from the vessel, the composition is changed and the interior of a furnace is contaminated by the abrupt increase of the temperature.
A maximum heating temperature in synthesization is preferably increased up to 1100xc2x0 C. so that a silicide reaction starts and synthesization is completed. Since a reaction temperature is different depending upon an amount of oxygen contained in the mixed powder, however, the maximum heating temperature is preferably increased to about 1300xc2x0 C. by taking the reduction of the oxygen content into consideration. When the temperature is increased to higher than 1300xc2x0 C., the sintering of a semi-sintered body formed by a silicide reaction proceeds and its crushing in a process III is made difficult and further free Si is melted as well as MSi2 grains are grown and coarsened by an eutectic reaction. Thus, there is obtained a structure in which MSi2 grains and Si grains irregularly disperse and as a result a silicide target having an intended crystal structure cannot be obtained. On the other hand, when the maximum heating temperature is not higher than 1000xc2x0 C., the silicide reaction does not start and synthesization is made impossible except the case that M is Ni. Thus, a more preferable temperature range is 1150-1250xc2x0 C.
Note, when the above maximum heating temperature is excessively high in the case M is Ni, sintering is liable to proceed as compared with the case M is other than Ni. Thus, the temperature is preferably increased up to about 800xc2x0 C. and more preferably in the range of 700-800xc2x0 C. only when Ni is used.
When a refractory metal silicide is synthesized as well as a semi-sintered body is formed in the process II, a vacuum furnace employed for heating is preferably, for example, a vacuum furnace using a high purity Mo heater or a high purity W heater and an insulator composed of a high purity refractory material, by which a semi-sintered body obtained by synthesization can be effectively protected from contamination caused by impurities from the heater and insulator.
In a process III, a refractory metal silicide semi-sintered body which is obtained by synthesizing silicide and has an atom ratio X of 2xe2x89xa6Xxe2x89xa64, is crushed or pulverized and a crushed powder is prepared. A powder lump in which free Si segregated to an aggregation of MSi2 formed in synthesization exists is finely crushed and uniformly dispersed by the crushing process. When this dispersing operation is no effected uniformly, since the dispersion of MSi2 and free Si is lowered, the structure and composition of a target are not uniformly arranged and a film characteristics are deteriorated, a crushing time is preferably not shorter than 24 hours. On the other hand, although the longer the crushing time, the more improved is a crushing efficiency, since productivity is lowered and an amount of contamination is increased by oxygen, the crushing time is preferably not longer than 72 hours. The maximum grain size of a powder obtained by the crashing is an important factor for obtaining a fine uniform structure defined by the present invention. Therefore, the maximum grain size is preferably not greater than 20 xcexcm and more preferably not greater than 15 xcexcm in order to obtain the structure defined by the present invention that MSi2 grains have an average grain size not greater than 10 m and free Si grains have a maximum grain size not greater than 20 xcexcm.
The crushing is preferably effected in vacuum or in an inert gas atmosphere similarly to the process I to prevent the contamination by oxygen. In particular, when a crushing mixer such as a ball mill or the like is used, contamination by impurities can be effectively prevented by carrying out mixing operation in a dry state using a ball mill having a main body the inside of which is lined with a high purity material and crushing mediums (balls) composed of a high purity material so that contamination caused by impurities from the crusher main body can be prevented.
Further, it is preferable that the following impurity removing process is followed by the process III to remove impurities contained in the crushed power such as oxygen, carbon etc. That is, the impurity removing process is a process for heating the crushed powder prepared in the process III and preparing a high purity powder and a high purity semi-sintered body by removing impurities such as in particular oxygen and the like therefrom. A heating temperature is preferably set to 1150-1300xc2x0 C. to effectively remove oxygen adsorbed to the crushed power. More specifically, when the heating temperature is less than 1150xc2x0 C., it is difficult to obtain a low oxygen target containing oxygen in a amount not greater than 200 ppm by volatilizing and removing oxygen as silicon oxide (SiO or SiO2). On the other hand, when the heating temperature exceeds 1300xc2x0 C., a problem arises in that free Si is greatly volatilized and lost, and it is difficult to obtain a target having a predetermined composition, and further a semi-sintered body is cracked, sintering proceeds and an amount of contraction increases, and the semi-sintered body cannot be hot pressed in the state as it is. Consequently, a more preferable temperature range is 1200-1250xc2x0 C.
In particular, when the heating temperature increases, since the semi-sintered body is liable to be cracked, it is preferable that the semi-sintered body is processed while applying a low pressing pressure thereto. The pressure is preferably in the range not greater than 10 kg/cm2.
Further, the above heating temperature is preferably held for 1-8 hours. When the holding time is shorter than 1 hour, oxygen is insufficiently removed, whereas when the time exceeds 8 hours, a long time is needed and productivity is lowered as well as a large amount of Si is volatilized and lost, and the dislocation of the composition of a silicide target increases. Thus, the holding time is more preferably set to the range of 2-5 hours.
A degree of vacuum is preferably set to not higher than 10xe2x88x923 Torr and further to not higher than 10xe2x88x924 Torr to more effectively reduce oxygen by volatilizing silicon oxide. A further deoxidizing effect can be obtained and a target containing a less amount of oxygen can be obtained in such a manner that after the degree of vacuum is adjusted, hydrogen is introduced into a heating furnace and the target is heated in a pressure-reduced hydrogen atmosphere.
A vessel into which the crushed powder is charged may have a shape and size equal to those of a compacting mold to be used in a sintering process such as a hot pressing or the like to be described later or may be formed to a size determined by taking an amount of contraction of a semi-sintered body caused by calcination into consideration. As a result, there can be obtained an advantage that a deoxidized semi-sintered body can be easily inserted into the compacting mold and a plurality of semi-sintered bodies can be simultaneously sintered, and productivity can be greatly improved. The vessel is preferably composed of a high purity material of Mo, W, Ta, Nb or the like to prevent the contamination of the crushed powder by impurities and thermal deformation.
The crushed powder charged into the vessel is preferably smoothed by a dedicated pattern and made to flat by moving the powder forward and backward and in rotation so that the deoxidized semi-sintered body can be hot-pressed in the state as it is.
In a process IV, a crushed powder prepared in the process III or a semi-sintered body having been subjected to the impurity removing process is subjected to a main sintering and densification or compaction. The crushed powder or semi-sintered body having been subjected to the impurity removing process whose Si/M atomic ratio is adjusted to 2-4 and which is composed of MSi2 and excessive Si is charged into the compacting mold and sintered and densified while setting a temperature and pressure at two steps.
The compacting mold to be used here is preferably a graphite compacting mold arranged such that, for example, a BN powder or the like having an exfoliation resistance at high temperature is coated on the inner surface of the mold with a spray or brush as a mold releasing agent and further a partition plate is applied onto the inside surface through a double-coated adhesive tape, adhesive or the like. The mold releasing agent is coated to prevent a compacting mold main body from being fused to the partition plate in hot pressing. The partition plate is provided to prevent the direct contact of the semi-sintered body with the mold releasing agent and isolate the former from the latter. As the partition plate, a refractory metal such as Mo, W, Ta, Nb etc. enduring high temperature in sintering and Ni, Ti etc. excellent in workability and processability is used by being formed to a thickness of 0.1-0.2 mm. When the partition plate is excessively thick, since its strength is increased, the formability of the plate is lowered when it is applied onto the compacting mold and workability is lowered as well as since the partition plate is adhered onto a sintered body, a long time is needed to remove it by grinding or the like. On the other hand, when the partition plate is too thin, since its strength is small, the plate is difficult to handle and workability is also lowered.
The fusion of the compacting mold with the partition plate is prevented as well as the mold releasing agent is not exfoliated and removed and the mixing of impurities contained in the mold releasing agent with a sintered body can be effectively prevented by coating the mold releasing agent on the inner surface of the mold and further using the compacting mold on which the partition plate is applied. In particular, even if BN is used as the mold releasing agent, the contamination of a target caused by inevitably contained impurities such as aluminium, iron etc. can be effectively prevented.
Next, sintering is carried out by applying a low pressing pressure of 10-50 kg/cm2 in a high vacuum not higher than 10xe2x88x923 Torr and increasing a temperature up to just below an eutectic temperature stepwise or at a small temperature increasing rate.
A pressing pressure is preferably set to 10-50 kg/cm2 at a first step because the pressure affects the remaining of aggregated silicon and the grain size of MSi2. When the pressing pressure is less than 10 kg/cm2, MSi2 grains grow as well as a composition is not uniformly distributed. On the other hand, when the pressure is not less than 50 kg/cm2, the ductile flow of free Si is suppressed and aggregated Si remains, and a structure in which Si is not uniformly dispersed is obtained. The pressure is more preferably 20-30 kg/cm2.
When sintering is carried out by increasing a temperature up to just below an eutectic temperature while applying a pressure, heating is preferably effected stepwise or at a low temperature increasing rate to suppress the growth of MSi2 grains. A temperature increasing width is preferably 20-200xc2x0 C. When the temperature increasing width is less than 20xc2x0 C., a long time is needed for sintering and productivity is lowered, whereas when the width exceeds 200xc2x0 C., MSi2 grains are grown by an abrupt temperature increases as well as a composition has an inclined distribution in a target plane due to the flow of free Si. Further, each temperature is preferably held for 0.1-3 hours. When the holding time is less than 0.5 hour, the temperature of a sintered body in a mold is not uniformly distributed, whereas when the time exceeds 2 hours, a long time is needed and productivity is lowered. Thus, it is more preferable that the temperature increasing width is set to the range of 50-100xc2x0 C. and the holding time is set to the range of 0.5-2 hours.
Further, when a heating rate exceeds 20xc2x0 C./minute in the heating effected at a low rate, MSi2 grains are coarsened. Thus, the heating rate is preferably set to not higher than 20xc2x0 C./minute. Further, when the heating rate is less than 3xc2x0 C./minute, since a long time is needed to sintering operation and productivity is lowered, it is preferably set to the range of 3-20xc2x0 C./minute and more preferably to the range of 5-10xc2x0 C./minute.
A final sintering temperature T is preferably set to just below an eutectic temperature, i.e., to the range of Tsxe2x88x9250xe2x89xa6T less than Ts. When, for example, W, Mo, Ti, Ta are used as M, the eutectic temperature Ts is 1400, 1410, 1330, 1385xc2x0 C., respectively. Note, the eutectic temperature Ts can be easily obtained by referring to literatures such as xe2x80x9cConstitution of Binary Alloysxe2x80x9d (Dr. phil. Max Hansen and Dr. Kurt Anderko; McGraw-Hill Book Company, 1958) and the like. When T is not higher than (Tsxe2x88x9250), pores remain and a desired high density target cannot be obtained, whereas when T is not less than Ts, free Si is melted and flows out from the compacting mold and a target with a dislocated composition is obtained.
Since a pressing pressure at a second step affects the density of a resultant sintered body, the pressure is preferably set to 200-500 kg/cm2. When the pressing pressure is less than 200 kg/cm2, a sintered body with a density not less than 99% cannot be obtained, whereas when the pressure is not less than 500 kg/cm2, a graphite compacting mold is liable to be broken. Thus, the pressing pressure is more preferably set to the range of 300-400 kg/cm2.
The pressing pressure is preferably applied in 1-5 hours after a final temperature is reached. When the period of time is less than 1 hour, the temperature of a semi-sintered body in a mold is not made uniform and when the pressing pressure is applied in this state, a problem arises in that a uniform density distribution and uniform structure cannot be obtained due to an irregular temperature distribution. On the other hand, when the time exceeds 5 hours, although the temperature of the semi-sintered body in the mold is completely made uniform, the holding of the semi-sintered body longer than this time lowers productivity. Thus, the holding time is preferably 2-3 hours.
Further, the pressing pressure is preferably held for 1-8 hours. When the holding time is not longer than 1 hour, many pores remain and a high density target cannot be obtained, whereas when it is not shorter than 8 hours, since densification does not further proceed, the manufacturing efficiency of a target is lowered. Thus, the holding time is more preferably 3-5 hours. The sintering for the densification is preferably carried out in vacuum to prevent the contamination caused by the mixture of impurities.
An intended sputtering target can be finally obtained by machining a resultant sintered body to a predetermined shape. At that time, it is preferable to finish the sintered body by a machining method which does not produce a surface defect on the surface of the target.
A high purity silicide thin film can be formed by effecting sputtering using the target. Further, various electrodes such as a gate electrode, source electrode, drain electrode and thin film for a semiconductor device and a thin film for wiring materials can be formed by subjecting the thin film to etching and the like.